1. Field of the Invention
This invention is directed to a method for the measurement of stress-strain relationships in thin films using substantially flat and parallel test surfaces with minimal width. This invention is particularly directed to a method for the measurement of stress-strain relationships in thin films with a thermo control means for temperature dependent measurements and to an apparatus for making the measurement.
2. Description of the Related Art
A wide variety of techniques have previously been used in the measurement of mechanical properties of thin films. Tensile tests are well-established and most-acceptable tests for measuring in-plane properties of thin films. However, the in-plane properties can be different from out-of-plane properties, and it is the latter which are often more relevant to the performance of the films in their intended applications. In addition, tensile experiments impose tensile stresses on samples, which promote fracture failure earlier in the deformation, and may mask observations of plastic flow. In addition, areas supporting the load in tensile tests decrease as loads increase, leading to instability with necking and non-uniform straining. As a result, reliable large-strain properties cannot be obtained from tensile tests.
Representative of the aforementioned tensile tests is bulge testing, which is a biaxial tensile test. There, both stresses and strains can only be roughly estimated through mathematical modeling because of the complex geometry of deformation.
Nano-indentation testing is a widely-used technique to study the mechanical properties of thin films. While simple in operation, expensive equipment has been required, and the non-uniform deformation results in complicated stress and strain fields, and also makes understanding and interpreting the measurements difficult. Even with the help of sophisticated computational models, no stress-strain relationships can be extracted from nano-indentation measurements.
In addition, methods mentioned above can not provide mechanical property information of thin films under higher strain-rate, i.e., a strain-rate greater than 100% per second.
An improved test and an apparatus were disclosed in U.S. Pat. No. 5,517,860, wherein stress-strain relationships for thin films can be measured in a plane-strain compression condition. The aforementioned U.S. Patent is herein incorporated by reference. However, the method and the testing apparatus provided by the aforementioned patent can only measure stress-strain relationships under strain rates up to about 100 per second (100% per second). Although it is a few orders of magnitude higher than conventional tensile tests which can only measure stress-strain relationships of thin films under strain-rates in a range from 10−3 to 10−1 (0.1 to 10%) per second, the strain-rate applied by the method and the testing apparatus provided by the aforementioned patent is still far below the strain rates expected in a real impact situation which is usually in a range of 102 (10,000%) per second or higher. It is therefore still in need for a testing method and an apparatus that can measure mechanical properties of thin films in strain rates close to the real impact situation. In addition, there are also needs to obtain film mechanical properties at temperatures other than ambient temperatures.